Different Types of Roof Trusses

Written by peter lancett

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Roof trusses help spread the weight of the roof to the exterior walls. (roof trusses inside house image by Adrian fortune from Fotolia.com)

Roof trusses are used to frame the roof during house construction or renovation. Typically, they are prefabricated from wood. These triangular structures support the rest of the interior and exterior materials that make up the roof, spreading the weight of the roof to the exterior walls.

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Bowstring Truss

Bowstring trusses span large distances, such as those required for the roof of a warehouse or aircraft hanger. Bowstring trusses resemble the arched shape of an archery bow. A bowstring truss consists of multiple beams forming the arch. Curved sheets of metal, or other material, form the arch of the roof over the truss. Sometimes, the lower sections of the bowstring truss go up at a slightly steeper angle than the other sections of the truss. This lets any water to run off more readily. An advantage of roofs built over bowstring trusses is that there is no need to cap the ridge at the top of the roof, saving time and money in the construction process. A ridge cap normally runs along the length of a roof, covering the seam where the materials forming each side of the roof join at the very top of the roof. Bowstring truss roofs are arched, so there is no point at the top where two sets of roofing materials meet.

Dual-Pitched Truss

A dual-pitched roof truss is a standard triangular shape. The point at the top of this triangular shape is known as the apex of the triangle, and the struts leading down from the apex are called the "upper chords" of the truss. The two upper chords are joined at their bottom ends to a horizontal beam, forming the base of the triangular shape, which is known as the "lower chord." In a dual-pitched truss, the upper chords are of differing lengths. This means that the shorter of the two upper chords has to go up at a steeper angle to the apex than the longer of the two upper chords. The result is that the apex of the truss is off-centre. Dual-pitched trusses are used to span large areas where there is a need for wide floor space beneath, unobstructed by support poles. The longer length and the smaller length bear the load of the roof equally, spreading it to the lower chord. A dual-pitched roof truss can span the same area as a truss with equal-length upper chords, but it uses less construction material.

Gambrel Roof Truss

Gambrel roof trusses have the characteristic silhouette of a barn roof seen from the front. A gambrel roof truss consists of two sets of upper cords, with the bottom set of upper chords longer than the top set of upper chords. The top set of upper chords meet at the apex of the truss. The length and steep angle of the bottom set of upper chords, as they rise up from the lower chord in this truss design, mean that the space beneath the roof becomes usable as living or storage space. This is because the steeply rising upper chords form a high wide space that does not get significantly narrower than the width of the house. When a gambrel roof is used as the design feature of a home, it allows the home to incorporate an upper floor, immediately beneath the roof. The steep angle of the bottom set of upper chords also gives extra flat space for windows and skylights, allowing more light into the upper floor of the house.